|The fans spinning up don't mean the computer is working. It only means the fans are getting 12 volts. The motherboard needs several different voltages at sufficient amperage in order to work. So it could be the power supply.|
But first, did anything happen to it or did you make any changes between the last time it worked OK and the first time it didn't? Could someone else in the household have messed with it?
Otherwise, with the AC cord removed, open the case and temporarily disconnect the drives and remove any unneeded add-on cards. Remove any USB devices you have connected. Then reconnect the AC cord and power it up. If you get the normal posting or logo screen then one of the disconnected items must be at fault.
If you still get nothing it's probably the motherboard. Sometimes the capacitors go bad, especially those around the cpu. They'll get kind of a puffy look to them like a soda can that's been in the freezer too long and they'll be some dried residue on the top indicating leakage.
And sometimes a bad power supply will ruin the motherboard as well so it could be both of them. But hopefully you'll be able to isolate the cause to something not too complicated.